The present invention relates to a fluid detection device and, particularly, but not exclusively, a fluid detection device which is used to detect the level of a hazardous fluid within a vessel.
Liquid level detection sensors are known and have application such as to detect the level of liquids in vessels. For example, liquid level detectors are used for over-fill protection in petrochemical tanks.
Liquid detection devices are known for this application which utilise the principle that a beam of light transmitted through glass can be re-directed at the boundary of the glass according to the refractive index of the surrounding medium. These liquid level detection devices usually comprise a glass prism and, when the glass surface is not in the liquid, light transmitted to the glass prism is reflected back internally to a photo detector, while if the surface of the prism is in a non-reflective liquid, the light is almost completely refracted out into the liquid. If a signal is received by the light detector, therefore, the probe is not in liquid. If a signal is not received, either the probe is in liquid or a fault has occurred.
In prior art liquid level detectors of this form, the detection prism, which is of varying degrees of complexity, is usually housed in a metallic xe2x80x9ctipxe2x80x9d, typically of stainless steel, and held in position by a hard-setting epoxy. Light emitters and receivers are glued or fixed in precise places relative to the glass prism and a rear body houses the processing electronics. The rear body is fixed to the tip and potted to produce the finished assembled xe2x80x9cprobexe2x80x9d (a term generally used for such devices). The internal electronics is also potted within the rear body. As such probes are often used with hazardous liquids, it is important that the electronics is protected from the hazardous liquid and any fumes.
The probe is usually mounted in a vessel (e.g. petrochemical tank) usually mounted extending through the wall of the vessel with the cable-end of a conductive cable connected to the electronics being external to the vessel.
There are a number of problems with present probe constructions. In particular, the epoxies used to house and seal the glass prism may be chemically attacked by many of the products handled by industry. This can lead to a dangerous situation where operating electronics may be exposed. Even the stainless steels used to construct the probe tips are frequently attacked.
This means that most probes are used only for a xe2x80x9cnormalxe2x80x9d liquid such as motor vehicle fuel. Even specially made probes using materials such as stainless steel and polytetrafluroethylene have limited use.
There are also a number of joints (electrical and mechanical) in the probe and this can lead to potential leak paths into the electronics.
Further, current probes are costly because of the complexity of assembly required and material costs.
According to one aspect of the present invention there is provided a fluid level detector comprising a light transmitter for transmitting light to an optical element, a light receiver for receiving light reflected from the optical element, electrical circuitry for detecting signals from the light receiver, and a glass housing substantially encapsulating the light receiver, light transmitter and electrical circuitry, whereby to protect the detector from hazardous environments.
Preferably, any type of glass material may be used to form the glass housing, and other transparent materials with high temperature capabilities, the suitable refractive index properties and also, if chemical resistance to hazardous chemicals, may be used. The term xe2x80x9cglassxe2x80x9d used in the preceding paragraphs should be taken to encompass such materials.
Preferably, the glass housing has an open end where an electrical cable connected to the electrical circuitry within the housing can exit. Preferably, in use, this open end is not placed within the liquid environment. Where the environment is a vessel, such as a fuel tank, the detector is preferably mounted through a passageway in the wall of the vessel, so that the open end of the glass housing is on the outside of the vessel and the rest of the glass housing extends within the vessel. This has the advantage that only the glass housing is exposed to any potential hazardous chemical. There are no joints and the only surface exposed to the liquid is glass. As glass is relatively inert, this liquid level detector may be used with almost any liquid with no risk of permeation through joints which may damage the operation of the unit.
The components within the glass housing (light receiver, electrical circuitry and light transmitter and any other components) are preferably potted with a potting compound e.g. epoxy. This potting provides further protection against access to the componentry, and also physically stabilises and supports the componentry so that it is protected from mechanical damage. Preferably, the potting material is introduced through the open end of the glass body after the componentry has been inserted into the glass housing, and when the potting compound has been introduced an end seal is provided about a cable exiting the open end of the glass housing.
The optical element is preferably provided by a portion of the glass housing itself. Preferably, the distal end of the glass housing body, opposite the open end, is formed as a glass prism for acting as the optical element.
One further problem that needs to be addressed in relation to the probe of the present invention, is that is there is a very large difference in coefficient of thermal expansion between any glass (very low expansion) and the typical potting compound such as epoxy (very high expansion). A liquid detector in accordance with the present invention is required to survive environments ranging from xe2x88x9240xc2x0 C. to +120xc2x0 C., and at this range the relative movement between potting compound and glass would be sufficient to crack the glass and delaminate the bond between the potting compound and the glass, unless further measures are taken.
A further preferred feature of the present invention, is the provision of a layer of compressible material between the potting compound and the glass housing. The compressible material is preferably a foam material.
The compressible material is preferably sufficiently compressible to absorb expansion and contraction differences between the potting material and the glass housing, due to temperature changes. Using such a compressible material preferably prevents the glass housing from breaking.
Preferably, during assembly of the liquid sensor, the foam material is mounted to the inside of the glass housing before potting compound is introduced.
According to another aspect of the present invention there is provided a fluid level detector comprising a light transmitter for transmitting light to an optical element via light transmission means, a light receiver for receiving light reflected from the optical element via said light transmission means, electrical circuitry for detecting signals from the light receiver, said light transmitter, light receiver and electrical circuitry being located remote from the optical element.
Preferably the optical element is formed by a prism-shaped portion of a glass housing which houses at least part of the light transmission means. Alternatively the optical element is formed by a prism-shaped portion of the light transmission means itself.
Typically the light transmission means is in the form of a pair of fibre optic cables being configured to transmit light from and to the light transmitter and light receiver, respectively.
According to a further aspect of the present invention there is provided a method of manufacturing a fluid level detector, comprising the steps of forming a glass envelope with an open end and an optical element formed within the wall of the glass housing, inserting through the open end a light transmitter for transmitting light to the optical element, a light receiver for receiving light reflected from the optical element, and electrical circuitry for detecting signals from the light receiver.
Preferably the method further comprises a step of introducing potting material through the open end of the housing, to pot the light transmitter, light receiver and electrical circuitry. More preferably the method also comprises a step of, before introducing the potting material, lining the inner surface of the glass housing with a compressible material which is sufficiently compressible to absorb expansion and contraction differences between the potting material and the glass housing, due to temperature changes.